Polydiacetylenes have been of interest in nonlinear optics for the very high values of their third-order susceptibilities .chi..sup.3 (.omega.) and it has been suggested that these materials be used in all-optical signal processing schemes based on intensity-dependent index of refraction n.sub.2 (proportional to .chi..sup.3 (.omega.)). U.S. Pat. No. 4,431,263 to Garito discusses the use of diacetylene species and polymers formed therefrom to provide nonlinear optic waveguide materials and U.S. Pat. No. 4,220,747 to Prezios et al. discusses crystalline diacetylene polymers as being useful as photoconductive materials.
The ability to generate, guide, modulate, and detect light in thin film configurations (thickness comparable to the wavelength) opens up possibilities for monolithic "optical circuits". It is important to emphasize that ability to form waveguides in different materials is crucial in further investigations and developments in the area of "integrated optics".
Polydiacetylenes are formed by a solid state reaction of adjacent monomer units in diacetylene single crystals which can be grown by a variety of approaches in desired macroscopic forms. Diacetylene obtained in the form of thin film single crystals and in the form of multilayer assemblies can be made using novel growth techniques, M. Thakur and S. Meyler, Macromolecules, 18, 2341 (1985); and Langmuir-Blodgett film balance respectively, or as well as other techniques. The solid state reaction of diacetylene monomers can be initiated by UV irradiation, heat, pressure, or high energy gamma-rays and results in single crystals of macroscopic planar dimensions up to a few centimeters. During all polymerization procedures mentioned above, polymerization of the total volume of material with no control of the thickness of the polymerized part is taking place.
Several conditions are required in order for an optical waveguide to be formed:
The thickness of the film representing the waveguide has to be well controlled. PA1 The index of refraction of the film representing the waveguide (n.sub.f) must be larger than indices of refraction of the substrate (n.sub.s) and the cover (n.sub.c). PA1 Uniform interface between the film and the substrate provides better conditions for propagation of light in a waveguide. PA1 radiation damage PA1 formation of donor and acceptor energy levels PA1 stoichiometric changes of the substrate.
Fabrication techniques that are used for making optical waveguides can be roughly classified into two types: in the first one a thin transparent layer is grown on a substrate of less refractive index; in the second, transparent substrate is submitted to some physical process, such as chemical diffusion or ion implantation, which increases its refractive index, as compared to the substrate.
The principle of ion implantation is rather simple. A collimated ion beam of well defined energy is sent into the substrate where an optical waveguide is to be formed. Most modifications to the substrate introduced by ion beams and discussed in the literature are related to the following physical phenomena:
Ion implantation is well adapted for making channel waveguides when for example helium, protons or lithium ions are used during implantation, and waveguides have been successfully made in fused quartz, lithium niobate, GaAs and ZnTe.